Is "andei" also used for "estive" in Portuguese?
For example I've noticed "andei pensando" being used in conversations and I suppose that this is an informal way of saying "estive pensando". Is that the case?

Question

Answer

Yes, it is the case! You can also use "andar" in other tenses implying repetition.

Ando gastanto muito. I've been spending a lot.
Ele anda muito cansado. He's been very tired.

Answer

Sometimes the verb "andar" is used as auxiliar verb too. And likewise the verb "estar", in these situations is used to indicate continuous action as in your example "andei pensando".

But is not necessarily an informal way, this use of the verb "andar" can be found even in formal situations/texts both for Brazil and Portugal. 
-  "andar + gerund" (as used in Brasil)
-  "andar + a + infinitive" (as used in Portugal)

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